Soldering alloy



Patented Nov. 12, 1940 UNITED STATES soLDERiNG-ALLov Emerson W. Kern,New York, N. Y., assignor to Bell Telephone. Laboratories, Incorporated,New York, N. Yr, a corporation of New York No Drawing. ApplicationNovember 6, 1937,

Serial No. 173,149 U 2 Claims.

This invention relates to alloys and their use and more particularly toalloys which are suitable for soldering to thin metal objects, such asmetallic films or coatings and fine wires.

Heretofore difiiculty has been experienced in soldering metal bodies tothin metallic films or coatings or fine wires with the soldering alloysknown to the art. It has been found to be particularly difficult tosolder a metallic member to a thin metal film or coating, for example,of silver or gold, and such an attempt often has resulted in thedestruction of the thin metal surface. Various expedients have beenemployed to prevent the destruction of the thin metal but none have beenwholly satisfactory.

The primary object of the invention is to prevent the destruction ofthin metallic films or fine Wires by soldering operations.

Another object of the invention is to inhibit solution of the film orWire component in the solder.

A further object of the invention is to form alloys for soldering whichdo not attack the surface to which a metal is soldered.

Other and further objects and features of the invention will be apparentfrom the following description of certain exemplary embodiments thereof.

A feature of the invention is an improved method of soldering a metalobject to a thin metallic coating, film, or filament by first saturatingor partially saturating the solder with the same metal as that of thethin metal part, thereby preventing destruction thereof by absorption inthe solder.

A study of the pertinent phase diagrams in connection with successfuloperations with certain soldering alloys indicates that there is a groupof low melting point metals (under 350 degrees C.) in which a secondgroup of higher melting point metals (above 700 degrees C.) are onlyslightly soluble and dissolve without destroying the fluidity of themetals of the first group. Exemplary of the first group are: lead, tin,bismuth, thallium and cadmium; and of the second group: silver, gold,platinum, copper and nickel. It, therefore, appears that where it isdesired to solder to thin bodies of metal of the higher-melting-pointgroup, a successful operation results when the solder comprises one ormore metals of the low-melting-point group saturated or partiallysaturated with the same metal as that of the thin body.

In carrying out the teachings of this invention it has been found thatby saturating a lead solder at working temperature with silver, theabove-described difficulties are overcome. By the use of a saturatedsolution of the lead-silver alloy, the solution of the thin film ofsilver at the working temperature of the solder is prevented. 5 Alead-silver solder having a 7 per cent silver content may be employed tosolder to a thin film of silver without dissolving the thin film.However, when the concentration of the silver is below 1 per cent, thesolder dissolves the thin 19 film.

In the case of a tin-silver solder at the working temperature of thesolder, the phase diagram shows a tin-silver compound of the compositionAgsSn. Experiments with a 7 per cent silver-tin solder, at whichcomposition silver is in the form of the above compound, indicate thatthe solution of the thin film of silver is inhibited to such an extentthat the alloy could be satisfactorily used for soldering to thin silverfilms. so

It appears that the above-noted difficulty in the use of ordinary solderis due to the thin silver surface being absorbed by the molten solder.In the lead-silver solder of this invention, since it is alreadysubstantially saturated with 2 silver, the silver surface does not flowinto the solder, and in the case of the tin-silver solder, the rate ofsolution is so inhibited by bringing the silver into equilibrium withthe tin-silver compound that successful soldering operations result.

Advantages in certain cases have been found by employing an alloy solderof lead and tin in the approximate ratio of 5 to 4 with from 1 to 13 percent silver. The proper amount of silver 35 is a variable determined bythe temperature at which the soldering is performed. A particularcomposition which produces good results for soldering a wire to a silverfilm is lead 50 parts, tin 40 parts and silver 10 parts. With thiscompo- 40 sition the soldering is done without destroying or harming thefilm.

Where it is desired to solder thin filaments or films of gold, alloyssimilar to those used for silver give good results. A lead-tin alloyhaving the lead and tin in the ratio of approximately 5 to 4 and thegold ranging from 1 to 13 per cent of the whole is satisfactory. Goldforms compounds with lead and tin as may be shown by 50 reference tophase diagrams. Once the solution has been saturated with gold, furtherrate of solution of the gold is inhibited so that a successful solderingoperation may be performed. Using an alloy of lead 49.7 per cent, tin39.8 per la cent and gold 10.5 per cent, a gold filament was solderedwithout harm to a gold film.

In soldering to thin films or filaments of copper similar means andmethods may be employed. A suitable lead or tin or lead-tin solder issaturated or partially saturated with copper at or near the solderingtemperature and the resulting alloy used to solder to the thin copperbody. The proportions of copper may be varied from 1 to 13 per cent withsatisfactory results.

A thin film of copper has been successfully soldered by employing alead-copper solution (eutectic 2 per cent copper) as the solder. Thethin film of copper was not harmed in this operation. However, when leadalone was used as the solder, the film was damaged by the copper goinginto solution in the lead.

In soldering to thin films of platinum in accordance with thisinvention, a lead-platinum solder with the platinum in the range of from1 to 13 per cent, is suitable. Employing a solder comprising 94 per centlead and 6 per cent platinum in soldering to a thin film of platinum onglass, a satisfactory joint is obtained without harm to the film.

While the invention has been described with respect to several specificillustrative embodiments thereof, it is to be understood that it is notlimited thereto but by the scope of the appended claims only.

What is claimed is:

1. A soldering composition composed of from 3 to 13 per cent of silver,54 to 48 per cent lead and 43 to 39 per cent tin.

2. A soldering composition for use on a surface of gold, silver, orplatinum, composed of 54 to 48 per cent lead, 43 to 39 per cent tin and3 to 13 per cent of a metal corresponding to the surface to which thecomposition is to be applied.

EMERSON W. KERN.

